Process for the isolation of clavulanic acid and of pharmaceutically acceptable salts thereof

ABSTRACT

A process for the isolation of clavulanic acid and pharmaceutically acceptable salts thereof, such as potassium clavulanate, from the aqueous fermentation broth of a clavulanic acid-producing microorganism comprises the microfiltration of the broth without prior treatment.

This application is a continuation of, and claims the benefit under 35USC 120 of, application Ser. No. 09/415,483, filed Oct. 8, 1999, nowU.S. Pat. No. 6,207,428; which is a continuation of, and claims thebenefit under 35 USC 120 of, application Ser. No. 09/113,790, filed Jul.10, 1998, now U.S. Pat. No. 5,985,625; which is a continuation of, andclaims the benefit under 35 USC 120 of, application Ser. No. 08/632,402,filed Apr. 10, 1996, now U.S. Pat. No. 5,780,274; which is a 371 of PCTInternational Application No. PCT/SI95/00002, filed Feb. 23, 1995.

TECHNICAL FIELD (IPC C 12 P 17/18, C 07 D 498104)

The present invention belongs to the field of pharmaceutical industryand relates to a novel and improved process for the isolation ofclavulanic acid and of pharmaceutically acceptable salts thereof fromthe fermentation broth of Streptomyces sp. P 6621 FERM P 2804.

TECHNICAL PROBLEM

There exists a constant need for a novel and improved process forpreparing pure clavulanic acid and pharmaceutically acceptable saltsthereof such as potassium clavulanate by the isolation from thefermentation broth obtained by means of a clavulanic acid-producingmicroorganism, in which process sophisticated conventional isolationmethods and chromatographic purification of the desired product would beavoided.

PRIOR ART

Clavulanic acid is the common name for(2R,5R,Z)-3-(2-hydroxyethylidene)-7-oxo-4-oxa-1-azabicyclo[3.2.0]heptane-2-carboxylicacid of the following formula

Alkali metal salts and esters thereof are active as inhibitors ofbeta-lactamases produced by some Gram positive as well as some Gramnegative microorganisms.

In addition to the action of inhibiting beta-lactamases, clavulanic acidand alkali metal salts thereof also have a synergistic action incombination with beta-lactam antibiotics of penicillin and cephalosporinseries. Therefore clavulanic acid and salts thereof are used in galenicpreparations to prevent the deactivation of beta-lactam antibiotics.Commercial preparations contain a more stable potassium salt ofclavulanic acid (the acid alone is rather unstable) in combination withamoxicillin trihydrate.

Clavulanic acid is prepared by the fermentation of a clavulanicacid-producing microorganism such as various microorganisms belonging todifferent Streptomyces strains such as S. clavuligerus NRRL 3585, S.jumoniensis NRRL 5741, S. katsurahamanus IFO 13716 and Streptomyces sp.P 6621 FERM P 2804.

The aqueous culture broth obtained after fermentation may be purifiedand concentrated according to conventional processes comprising e.g.filtration and chromatographic purification as illustrated in GB1,508,977 prior to the extraction of the aqueous solution with theorganic solvent to obtain a solution of impure clavulanic acid in anorganic solvent.

GB 1,508,977 teaches, inter alia, that salts of clavulanic acid may beobtained by the adsorption of the clavulanate anion in the filteredbroth onto an anionic exchange resin and are eluted therefrom with anelectrolyte, the solution formed is desalted and then the solvent isremoved. This process may be used to achieve acceptable yields of thedesired substance, yet it requires sophisticated purifications bychromatographic methods and the use of resin columns demands importantinvestments, which limits production operations on a large scale.

GB 1,543,563 discloses a modified fermentation process using the strainS. clavuligerus NRRL 3585, wherein the pH value of the medium ismaintained in the range between 6.3 and 6.7 and thus the yield of thedesired compound increases. Salts of clavulanic acid such as potassiumclavulanate are prepared by re-salting from lithium clavulanate, wherebythe desired compound is also purified.

EP-A-0 026 044 illustrates the use of tert-butylamine salt of clavulanicacid as a useful intermediate in the preparation of clavulanic acid. Thesalt is known from BE 862 211, but only as an ingredient inpharmaceutical formulations.

EP-B-0 182 522 discloses a method of preparing clavulanic acid by thefermentation of microorganism S. clavuligerus. An important improvementof the process was achieved by the addition of a carbon source such asglycerol into the fermentation medium in the course of the processeither continually or intermittently, whereat it is very important thatthe carbon level is maintained in a sufficiently low concentration,namely under 0.5% (w/v) and by no means exceeds 2%. The Examplesillustrate that the essential improvement of the increased yield ofclavulanic acid was observed when the carbon source was added duringfermentation. It is stated that the concentration of clavulanic acid inthe fermentation broth after 160 hours was about 1400 μg/ml, this beinga noticeable improvement over previous processes.

A further improvement was also a novel process of purifing clavulanicacid from solution as its lithium salt. However, to achieve a higherpurity of lithium clavulanate a concentrated solution of another lithiumsalt such as lithium chloride was added. The obtained recrystallizedlithium clavulanate could be further purified and then optionallyconverted into other salts such as potassium clavulanate, in a mannerknown from the above literature.

The mycelium, proteins and other solids are removed by known methodssuch as centifugation or filtration with a possible prior treatment ofthe fermentation broth with a selected aggregation agent to aggregatethe mycelium and thus make possible an easier filtration. The filteredfermentation broth is further treated with ion exchange resins or byprecipitation with a solvent such as acetone in order to remove proteinsand the precipitate is separated by repeated centrifugation andfiltration. This separation of the mycelium, proteins and otheraccompanying particles in the suspension originally present in thefermentation broth, is time-consuming and requires several workingsteps.

These time-consuming methods of removing the mycelium, proteins andother suspended particles and subsequent isolation from the obtainedtransparent fermentation broth as well as preparation of pure clavulanicacid and salts thereof were avoided in a manner as disclosed in thepublished EP-A-0 385 552 and EP-A-0 387 178.

The whole process comprises three steps i.e. purifying the fermentationbroth of the mycelium, proteins and other solid particles, purifying theclavulanic acid present in an impure form in the broth of a purifiedfiltrate of Streptomyces clavuligerus by using one of the primary,secondary or tertiary amines forming stable intermediary salts ofclavulanic acid, whereby the major part of accompanying impurities inclavulanic acid are separated, and as the last step, the conversion ofintermediary amine salts of clavulanic acid (of 85% purity) into thedesired alkali metal salt such as potassium clavulanate.

The first step is disclosed in more detail in EP-A-0 385 552, whereinfrom the aqueous culture broth obtained by the fermentation of themicroorganism Streptomyces clavuligerus, by means of a physico-chemicalprocess of coagulation-flocculation, the mycelium, proteins and othersolid particles are removed. The flocculi obtained in this process aresufficiently large and compact so that an easy sedimentation andseparation is made possible, which is best achieved by using rollingsieves. Thus a transparent broth is obtained, which may be optionallyconcentrated by reverse osmosis.

In this manner a purified fermentation broth is obtained, theconventional purifying methods such as centrifugation, adsorption onactive carbon, filtration with coadjuvants etc. having been avoided.

In all known processes it is also necessary (which is different from thedisclosed flocculation method) that the purified broth of the culture istreated by means of various processes of deproteinization and ionexchange, which causes significant total losses in the final yield ofthe desired substance. In contrast to well-known methods, the totalyields in the flocculation method amount to 85 to 90%.

The disclosed method of coagulation-flocculation from the fermentationbroth of Streptomyces clavuligerus is based upon adding an inorganicelectrolyte into the broth culture to increase the coagulant action,applying the inorganic coagulant as initiator of the coagulation processunder stirring and at a pH value of the medium between 6 and 8, addingan organic electrolyte when the flocculation begins, and then separatingthe obtained flocculi from the fermentation broth using rolling sieves,or filtration and, optionally, when flocculation takes place in thepresence of a water-immiscible solvent, decanting the phases, separatingthe flocculi and, optionally, concentrating the liquid by reverseosmosis or evaporation.

EP-A-0 562 583 discloses the use of salts of clavulanic acid withorganic diamines such as N,N′-diisopropylethylenediammoniumdiclavulanate as useful intermediates for the isolation and preparationof pure clavulanic acid or alkali metal salts thereof such as potassiumclavulanate from ethyl acetate extract obtained after the solventextraction of the obtained aqueous culture broth formed afterfermentation, wherein clavulanic acid is present.

TECHNICAL SOLUTION

The aim of the invention is to improve the process of isolation ofclavulanic acid from the fermentation broth obtained by means of aclavulanic acid-producing micro-organism such as Streptomyces sp. P 6621FERM P 2804, in which process time-consuming conventional methods ofremoving the mycelium, proteins and other suspended solid particlespresent in the aqueous culture broth would be avoided, followed by thepreparation of salts of high purity such as potassium clavulanate.

Suitable salts according to the present invention are pharmaceuticallyacceptable alkali metal and alkaline earth metal salts such as sodium,potassium, calcium and magnesium salts. Among these salts sodium andpotassium salt, especially potassium salt are the most suitable.

The present invention is generally useful for purifying fermentationbroths obtained by means of a clavulanic acid-producing microorganism.

It is evident from the above Prior Art that known processes havecomprised time-consuming isolation methods and only EP-A-0 385 552discloses an improved process, wherein a completely transparent broth isobtained. However, it is a disadvantage of this process that, in orderto achieve the desired aim, several reagents such as inorganicelectrolytes, coagulants, organic polyelectrolytes have to be used andthat flocculation, sedimentation or filtration of the fermentation brothrequire a relatively long production time, which affects the purity ofthe desired product.

On page 2, column 2, lines 22 to 35, some possibilities of purifying thefermentation broth are given, yet said methods would lead to asignificant decrease of the yield of clavulanic acid. It is furtherstated that the use of several sophisticated techniques in the processof isolation and purification such as ultrafiltration and reverseosmosis would not simplify the process because the use of those methodswould require prior filtrations on active carbon or ionic resins.

Contrary to these statements it has surprisingly been found that it maybe possible to avoid the use of several reagents as used in the processdisclosed in EP-A-0 385 552, as well as other time-consuming ways ofpurification of the aqueous fermentation broth, which ways are disclosedin the literature, when according to the present invention amicrofiltration method is used, wherein the mycelium, a major part ofproteins (at least 80% of those present in the broth) and othersuspended particles are removed.

To this purpose there a multi-stage device for continuousmicrofiltration is used, which makes possible to carry out the processof separating the mycelium and aqueous filtrate in a dwell time of lessthan half an hour, the device being composed of several (five) seriallyconnected independent segments (filtration loops). Each segment has itsown circulation pump which permits the desired velocity of thefermentation broth (5 to 8 m/s) through channels of ceramic filteringelements having a pore size of 0.05 μm. In the microfiltration processtaking place at a temperature between about 20° C. and 40° C. (thetemperature should not exceed 40° C.), tangentional velocities areregulated in such a way that in a solid fraction the molecules of amolecular weight over 30,000 are retained. In such a way we succeeded inremoving about 80 to 90% of the proteins present. The mycelium separatedin the microfiltration process was also washed with water in order toincrease the yield of clavulanic acid in the combined filtrate. By thedisclosed method of purifying the aqueous fermentation broth bymicrofiltration, over 95% of clavulanic acid are retained in thepurified aqueous phase, which even exceeds the results of theflocculation method of EP-A-0 385 552 and represents a furtherimprovement of the present invention.

After microfiltration the filtrate may be optionally purified by anultrafiltration process. The purpose of that purification is to separatea major part of the remaining protein impurities and other accompanyingimpurities having a higher molecular weight than clavulanic acid. Insuch a way undesired impurities are successfully removed, which mayprecipitate upon extraction with a water-immiscible organic solvent,whereby the colouring of the aqueous filtrate obtained afterpurification with microfiltration is essentially reduced and the purityof the desired product is further improved.

In the ultrafiltration device a polymeric membrane having a highresolution rate of about 20,000 daltons (between 10,000 and 30,000daltons) is used. The process is carried out continuously so that thedwell times are as short as possible, and over two serially connectedultrafiltration devices (which increases the separation selectivity ofimpurities and clavulanic acid); by adding pure washing water as well asby counter-current conveying of the retentate (aqueous phase) obtainedin the ultrafiltration process, the losses of clavulanic acid in theaqueous phase are reduced to less than 5%.

The combined aqueous filtrate is then concentrated in a reverse osmosisdevice at a temperature about room temperature to about ⅕ of theoriginal volume to obtain a concentrated aqueous phase of impurecalvulanic acid and then the obtained concentrate (retentate) isdirectly extracted at temperatures between 15° C. and 25° C. (extractionmay also take place at a temperature under 15° C.) with awater-immiscible organic solvent such as ethyl acetate, in an acidicmedium in the pH range of the medium between 1 and 3, which is adjustedby adding inorganic acid such as sulturic acid. Besides ethyl acetate,also other water-immiscible organic solvents may be used such as methylacetate, methyl isobutyl ketone or n-butyl alcohol.

Since by the microfiltration process we succeeded to remove the myceliumand a major part of the proteins present, a direct extraction of thepurified and concentrated aqueous fermentation broth with an adequatewater-immiscible organic solvent such as ethyl acetate is possiblewithout the use of time-cosuming purification methods as applied inknown ways disclosed in the above literature and there is avoided theuse of additional reagents as applied in the coagulation-flocculationmethod for the purifying of the fermentation broth. Thus, in addition tothe above improvement, the process according to the invention alsoprovides a reduction of costs of the method of broth purification. Toavoid a denaturation of the remaining proteins in the aqueous phaseconcentrate because of an interaction with the organic solvent orsulfuric acid in the course of the extraction of the impure clavulanicacid from the aqueous phase into the organic phase, it is best to carryout the extraction in a series of centrifugal extractors, wherein in oneof them, namely in a self-emptying centrifugal separator, the separatedproteins are removed simultaneously and continuously.

In the obtained extract of the impure clavulanic acid in awater-immiscible organic solvent there may be also, presentwater-soluble impurities such as various decomposition products ofclavulanic acid that are more polar than clavulanic acid alone, hencewater-soluble impurities are removed by washing the combined organicphase with water. In this manner a purified extract of clavulanic acidin organic phase such as ethyl acetate extract is obtained.

Clavulanic acid may be isolated from the organic phase and purified soas disclosed in the process of our EP-A-0 562 583. The best way ofisolating clavulanic acid as described in this patent application iscarried out by a reaction of the ethyl acetate extract of the clavulanicacid with N,N′-diisopropylethylenediamine at a temperature of about roomtemperature and by a subsequent conversion of the obtained intermediaryN,N′-diisopropylethylenediammonium diclavulanate with potassium 2-ethylhexanoate in an aqueous isopropanol solution and at room temperature toobtain potassium clavulanate, which is isolated with a high purity.

Now it has been found that the preparation of the intermediaryN,N′-diisopropylethylenediammonium diclavulanate is best carried out insuch a way that in the reaction between a water-immiscible organic phasesuch as ethyl acetate extract of the clavulanic acid andN,N′-diisopropylethylenediamine there is used an organic phase wherefromwater has been completely removed since already small water amounts maydisturb the preparation of the intermediary salt as the separated saltdissolves in the water present in the organic phase and pitchedby-products may be obtained, which makes the drying more difficult.

If water is completely removed from the organic phase, the stability ofthe organic phase or of the extract increases since it is well-knownthat the stability of clavulanic acid in aqueous solutions and in anacidic pH medium in the extraction process is very poor. Therefore forthe drying of the organic phase such as ethyl acetate extract ofclavulanic acid, drying in a rectification column (principle offractional distillation) in vacuo was used because of the poor stabilityof the intermediary salt at higher temperatures. It is an essentialfeature of this method that the organic phase such as ethyl acetate andwater form an azeotrope having a minimum boiling point and thus theorganic phase such as ethyl acetate extract is completely dried in thedisclosed manner. Thus the organic phase such as ethyl acetate extractsalways has a water content of less than 0.1 vol. %, averagely from 0.03to 0.05 vol. %. The completely anhydrous organic phase such as ethylacetate extract of clavulanic acid is then, within a very short dwelltime, concentrated by evaporation to a {fraction (1/20)} of the originalvolume to be subsequently reacted with N,N′-diisopropylethylenediamine.

The subsequent reaction of N,N′-diisopropylethylenediammoniumdiclavulanate with potassium 2-ethyl hexanoate to potassium clavulanateof high purity may be carried out as disclosed in our EP-A-O 562 583 andbest in such a way as disclosed in the Examples and illustrated by theabove improvements.

The invention is illustrated but in no way limited by the followingExamples.

EXAMPLE 1

Continuous Preparation of a Concentrate of Ethyl Acetate ExtractContaining Impure Clavulanic Acid

An aqueous fermentation broth (10,000 l) obtained by the fermentation ofthe microorganism Streptomyces sp. P 6621 FERM P 2804 (concentration ofclavulanic acid amounted to 3580 mg/l) was added to a 33% aqueoussolution (5 l) of sulfuric acid in a vessel (capacity 50 m³) understirring and cooling so that the pH value of the medium was maintainedbetween 5.8 and 6.2. Then the broth was continuously added to amicrofiltration device with a flow rate of 1200 l/h, which device wascomposed of five serially connected segments. Each segment had its owncirculation pump to provide for the velocity of the fermentation broththrough the channels of ceramic filtering elements with a pore size of0.05 μm to be 8 m/s. By the microfiltration process, wherein care wastaken that the temperature did not exceed 40° C., the mycelium and amajor part of proteins and of other suspended solid particles wereremoved.

The separated solids were washed with water having the flow rate of 300l/h and then the combined filtrate (permeate) after microfiltration wasadded continuously with the flow rate of 1500 l/h into a reverse osmosisdevice, wherein the permeate was concentrated to ⅕ of the originalvolume.

To the concentrate (retentate) obtained after the reverse osmosis with aflow rate of 300 l/h, a 33% aqueous solution (4 l/h) of sulfuric acidwas added so that the pH value of the medium was, maintained between 1.5and 2.0, then ethyl acetate was added at a flow rate of 900 l/h toextract an acidic retentate at room temperature in the countercurrent ina series of five centrifugal extractors, whereat in thesecond—self-emptying—centrifugal separator the still remaining separatedproteins were simultaneously removed.

The combined ethyl acetate extract from the series of the centrifugalextractors was washed in the first centrifugal extractor withdemineralized water having a flow rate of 30 l/h and thus the stillremaining water-soluble impurities were removed.

The ethyl acetate extract having the flow rate of 900 l/h was dried invacuo at the temperature of 30° C. in the rectification column so thatthe water content of 0.03 vol. % was achieved, then the extract wasevaporated in a thin-layer evaporator in vacuo at the temperature of 30°C. to a {fraction (1/20)} of the original volume. The obtainedconcentrated ethyl acetate extract (concentration of the impureclavulanic acid amounted to 50 g/l) having the flow rate of 45 l/h wasdecolourized by a continuous addition of active carbon (0.45 kg), themixture was stirred for 30 minutes and then the carbon was filtered offfrom the suspension of the concentrate of the ethyl acetate extract on apressure filter under nitrogen pressure of 1 bar to obtain a dryconcentrate (45 l) of the ethyl acetate extract containing impureclavulanic acid.

EXAMPLE 2

Preparation of N,N′-diisopropylethylenediammonium Diclavulanate

To a dry concentrate (45 l) of ethyl acetate extract obtained in thecontinuous process of Example 1 (the clavulanic acid content amounted to50 g/l), N,N′-diisopropylethylenediamine (1.4 l) was added for 5 minutesunder vigorous stirring at the temperature of 25° C. The obtainedsuspension was filtered off, the obtained crystals were resuspended inacetone (45 l) and, at stirring and cooling the suspension at atemperature under 10° C., crystals of the desired substance wereseparated, which crystals were filtered off, washed with acetone anddried in vacuo at the temperature of 30° C. Crystals ofN,N′-diisopropylethylenediammonium diclavulanate (3.3 kg; the clavulanicacid content amounted to 60%) were obtained.

EXAMPLE 3

Preparation of Potassium Clavulanate

N,N′-diisopropylethylenediammonium diclavulanate (3.3 kg) from Example 2was dissolved in an isopropanol/water mixture (82.5 l; the water portionamounted to 1.5%) and to the obtained solution active carbon (1.5 kg)and potassium 2-ethyl hexanoate (0.5 l; 2 M) were added for 30 minutesunder stirring at room temperature. Then the carbon and the obtainedprecipitate were filtered off. To the obtained filtrate (80 l) asolution (6 l) of potassium 2-ethyl hexanoate (2 M) in isopropanol wasadded for 20 minutes during stirring at room temperature. The obtainedsuspension was then stirred under cooling at a temperature between 0° C.and 5° C. for another 2 hours, then the separated crystals were filteredoff, washed with isopropanol and acetone and dried in vacuo at thetemperature of 30° C. Potassium clavulanate (2 kg; USP grade, theclavulanic acid content 80.6%, determined by HPLC method) was obtained.

What is claimed is:
 1. In a process for the isolation of clavulanic acidor a pharmaceutically acceptable salt thereof from an aqueousfermentation broth of a clavulanic acid-producing microorganism, theimprovement comprising, as a first step, multi-stage continuousmicrofiltration of the broth at a broth temperature between 20° C. and40° C., thereby removing substantially all of the mycelium and othersolid particles and a major part of the suspended proteins to produce amicrofiltration filtrate containing the clavulanic acid.
 2. Theimprovement of claim 1 where the microfiltration is carried out at abroth pH between 5.8 and 6.2.
 3. The improvement of claim 1 where themicrofiltration is through filter elements having a pore size of 0.05μm.
 4. The improvement of claim 3 where the microfiltration is at a flowvelocity between 5 m/sec and 8 m/sec.
 5. The improvement of claim 1further comprising, as a second step, concentration of themicrofiltration filtrate by reverse osmosis.
 6. The improvement of claim1 further comprising, as a second step, ultrafiltration of themicrofiltration filtrate.
 7. The improvement of claim 6 where theultrafiltration is continuous ultrafiltration using a semi-permeablemembrane having a resolution rate between 10,000 and 30,000 daltons. 8.The improvement of claim 6 further comprising, as a third step,concentration of the ultrafiltration filtrate by reverse osmosis.
 9. Theimprovement of claim 1 where the microfiltration is at a flow velocitybetween 5 m/sec and 8 m/sec.
 10. A process for the preparation of aconcentrated organic extract of clavulanic acid from an aqueousfermentation broth of a clavulanic acid-producing microorganism, theprocess comprising the steps of: (a) multi-stage continuousmicrofiltration of the broth at a broth temperature between 20° C. and40° C., thereby removing substantially all of the mycelium and othersolid particles and a major part of the suspended proteins to produce amicrofiltration filtrate containing the clavulanic acid; (b) optionally,ultrafiltration of the microfiltration filtrate to produce anultrafiltration filtrate containing the clavulanic acid; (c)concentration of the microfiltration filtrate from step (a) or theultrafiltration filtrate from step (b) to produce a retentate containingthe clavulanic acid; (d) extraction of the retentate with awater-immiscible organic solvent to produce an organic phase containingthe clavulanic acid; (e) drying of the organic phase containing theclavulanic acid; and (f) concentration of the organic phase containingthe clavulanic acid to prepare a concentrated organic extract ofclavulanic acid.
 11. The process of claim 10 where the microfiltrationis carried out at a broth pH between 5.8 and 6.2.
 12. The process ofclaim 10 where the microfiltration is through filter elements having apore size of 0.05 μm.
 13. The process of claim 12 where themicrofiltration is at a flow velocity between 5 m/sec and 8 m/sec. 14.The process of claim 10 where optional step (b) comprises continuousultrafiltration using a semi-permeable membrane having a resolution ratebetween 10,000 and 30,000 daltons.
 15. The process of claim 10 wherestep (c) comprises concentration of the microfiltration filtrate fromstep (a) or the ultrafiltration filtrate from step (b) by reverseosmosis.
 16. The process of claim 10 where step (d) comprisescountercurrent extraction of the retentate at a pH between 1 and 3 witha water-immiscible organic solvent.
 17. The process of claim 16 wherethe pH value of the retentate is adjusted with sulfuric acid.
 18. Theprocess of claim 17 where the retentate is extracted successively in aseries of centrifugal extractors.
 19. The process of claim 10 where theorganic solvent is selected from ethyl acetate, methyl acetate, methylisobutyl ketone, and n-butanol.
 20. The process of claim 19 where theorganic solvent is ethyl acetate.
 21. The process of claim 10 where step(e) comprises drying the organic phase in vacuo using a rectificationcolumn.
 22. The process of claim 21 where the organic phase is dried toa water content below 0.1%.
 23. The process of claim 22 where theorganic phase is dried to a water content between 0.03% and 0.05%. 24.The process of claim 10 further comprising decolorization of the organicphase.
 25. The process of claim 24 where the decolorization is carriedout with activated carbon.
 26. The process of claim 10 where themicrofiltration is at a flow velocity between 5 m/sec and 8 m/sec.